An optical communication system (hereinafter referred to as a WDM communication system), based on wavelength division multiplexing (WDM), communicates a large amount of data to satisfy increasing communication demands. WDM is a communication means that concurrently transmits different pieces of information by multiplexing optical signals having a plurality of different wavelengths.
The WDM communication system includes a network optical transfer device having a wavelength selective switch and transmits a plurality of pieces of information to desired output destinations. For example, the wavelength selective switch multiplexes received optical signals, and demultiplexes the multiplexed optical signals into optical signals of a given wavelength. The wavelength selective switch thus switches transfer paths of information.
FIG. 15A is a side view of a wavelength selective switch of related art. As illustrated in the side view of FIG. 15A, the wavelength selective switch includes a plurality of input ports, each port receiving an optical signal, and a plurality of output ports, each port outputting an optical signal. The wavelength selective switch includes a first optical system 100. The first optical system 100 includes a lens which collimates an optical signal input via an input port and an optical signal to be output via an output port.
FIG. 15B is a top view of the wavelength selective switch of related art. Upon receiving multiplexed optical signals having m wavelengths of “wavelengths λ1-λm” as illustrated in FIG. 15B, the first optical system 100 in the wavelength selective switch collimates the optical signal input via the input port, and directs the optical signal to a wavelength demultiplexer 200. The wavelength demultiplexer 200 includes a diffraction grating, and demultiplexes the input optical signal according to wavelength. More specifically, the wavelength demultiplexer 200 wavelength-demultiplexes the optical signal into m optical signals in an x axis direction. A second optical system 400 collects each optical signal demultiplexed by the wavelength demultiplexer 200 according to wavelength, and directs the optical signals to a micro-electromechanical system (MEMS) array 500. The MEMS array 500 includes m MEMS mirrors corresponding to the wavelengths.
The MEMS array 500 rotates the MEMS mirror about the x axis. The MEMS array 500 reflects an optical signal by changing the mirror's angle so that the optical signal of each wavelength is output from a given output port. The second optical system 400 in the wavelength selective switch collimates the optical signal reflected by the MEMS mirror, and outputs the collimated optical signal via a given output port through the wavelength demultiplexer 200. The wavelength selective switch may be set to output the optical signal at a given loss on a per-wavelength basis by rotating the MEMS mirror by a slight angle around the X axis or the Y axis. Solid lines represent optical paths of the optical signals, and double-arrow-headed lines represent the direction of travel of light.
Referring to FIGS. 15A and 15B, the wavelength selective switch includes a plurality of input ports and a plurality of output ports, and a known wavelength selective switch having a single input port and a single output port. Also known is a wavelength selective switch having a single input port and a plurality of output ports (drop-type wavelength selective switch). Another known wavelength selective switch includes a plurality of input ports and a single output port (add-type wavelength selective switch). Still another known wavelength selective switch includes no MEMS mirror and switches output ports by switching polarization directions of optical signals on a per wavelength basis.
A plurality of wavelength selective switches are typically used in an optical transfer device over a network. Since the wavelength selective switch is fabricated of a spatial optical system, a device including a plurality of wavelength selective switches is difficult to miniaturize. If a reconfigurable optical add/drop multiplexer (ROADM) is used in an optical transfer device at a branch point over a ring network, an add-type wavelength selective switch is used for each of the east-west and west-east directions. If a dynamic optical add/drop multiplexer (DOADM) adding or dropping an optical signal having a given wavelength is used, one add-type wavelength selective switch and one drop-type wavelength selective switch are used.